Ecobee is a very good product. It's a Canadian company, so that's always nice. Anyway, I'm pretty familiar with smart thermostats. I currently have the newest generation of Nest installed, more because I like the looks and manual interface better than Ecobee. The only way nest is somewhat superior is in the learning function but Ecobee has remote sensors which are very useful if you have a larger house. We recently downsized so the sensor is not an issue for me. The other thing with the sensor is that although a lot of people love it, If you have a number of people in various rooms, it will just average it out. Personally, if I'm in a room (basement) I just use echo to turn up the heat from wherever I am, which is basically what Ecobee is doing. It would be great to have a true smart zone system where heat could be diverted to whatever room you were instead of just increasing the house temperature - but those are big $.
As far as the Ecobee 4 goes, I THINK I would tend to prefer the 3 or 3 Lite, buy the sensor package and then install a number of dots throughout the house. The problem that's been reported with the Ecobee 4 is that it does not utilize ESP (Echo Spatial Perception) so if you an echo or echo dot within range - let's say an Echo in the kitchen and Ecobee 4 in a nearby hallway, The Ecobee will always respond when you're trying to communicate with the Echo. However you could defeat this if you changed your Echo to a different wake name "Echo" or "computer" but that ould be an issue for some.
Also, you're probably going to have more Dots or Echos in the house to control the Ecobee. If we're in the bed and we want to change the temperature to something other than the pre set temperature, you'd have to get up and move to wear the Ecobee could hear you.
There is no doubt that the Ecobee is an excellent product that will serve you well.